Problem: Which of these numbers is prime? ${4,\ 23,\ 35,\ 63,\ 69}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 4 are 1, 2, and 4. The factors of 23 are 1 and 23. The factors of 35 are 1, 5, 7, and 35. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 69 are 1, 3, 23, and 69. Thus, 23 is a prime number.